Deck watches were used on naval vessels as a secondary chronometer-grade timepiece to transport time around the ship, & in the days before satellite-based global positioning systems, for navigation. Before GPS, plotting a ship’s position required precise astronomical observations & a knowledge of the exact time in Greenwich, England, (or, alternatively, at the ship’s home base). Deck watches were synchronized on a daily basis with the ship’s main chronometer, a large wooden-boxed timepiece that was generally located on the bridge.